// Copyright Epic Games, Inc. All Rights Reserved.

#include "StaticMeshBuilder.h"

#include "BuildOptimizationHelper.h"
#include "Components.h"
#include "Engine/StaticMesh.h"
#include "IMeshReductionInterfaces.h"
#include "IMeshReductionManagerModule.h"
#include "Logging/StructuredLog.h"
#include "MeshBuild.h"
#include "MeshDescriptionHelper.h"
#include "Misc/ScopedSlowTask.h"
#include "Modules/ModuleManager.h"
#include "PhysicsEngine/BodySetup.h"
#include "StaticMeshAttributes.h"
#include "StaticMeshOperations.h"
#include "StaticMeshResources.h"
#include "Math/Bounds.h"
#include "NaniteBuilder.h"
#include "Rendering/NaniteResources.h"
#include "Interfaces/ITargetPlatform.h"
#include "RenderMath.h"

DEFINE_LOG_CATEGORY(LogStaticMeshBuilder);

void BuildAllBufferOptimizations(
	struct FStaticMeshLODResources& StaticMeshLOD,
	const struct FMeshBuildSettings& LODBuildSettings,
	TArray< uint32 >& IndexBuffer,
	bool bNeeds32BitIndices,
	const FConstMeshBuildVertexView& BuildVertices
);

FStaticMeshBuilder::FStaticMeshBuilder()
{

}

static bool UseNativeQuadraticReduction()
{
	// Are we using our tool, or simplygon?  The tool is only changed during editor restarts
	IMeshReduction* ReductionModule = FModuleManager::Get().LoadModuleChecked<IMeshReductionManagerModule>("MeshReductionInterface").GetStaticMeshReductionInterface();

	FString VersionString = ReductionModule->GetVersionString();
	TArray<FString> SplitVersionString;
	VersionString.ParseIntoArray(SplitVersionString, TEXT("_"), true);

	bool bUseQuadricSimplier = SplitVersionString[0].Equals("QuadricMeshReduction");
	return bUseQuadricSimplier;
}


/**
 * Compute bounding box and sphere from position buffer
 */
static void ComputeBoundsFromPositionBuffer(const FPositionVertexBuffer& UsePositionBuffer, FBoxSphereBounds& BoundsOut)
{
	// Calculate the bounding box.
	FBounds3f Bounds;
	for (uint32 VertexIndex = 0; VertexIndex < UsePositionBuffer.GetNumVertices(); VertexIndex++)
	{
		Bounds += UsePositionBuffer.VertexPosition(VertexIndex);
	}
	
	// Calculate the bounding sphere, using the center of the bounding box as the origin.
	FVector3f Center = Bounds.GetCenter();
	float RadiusSqr = 0.0f;
	for (uint32 VertexIndex = 0; VertexIndex < UsePositionBuffer.GetNumVertices(); VertexIndex++)
	{
		RadiusSqr = FMath::Max(	RadiusSqr, ( UsePositionBuffer.VertexPosition(VertexIndex) - Center ).SizeSquared() );
	}

	BoundsOut.Origin = FVector(Center);
	BoundsOut.BoxExtent = FVector(Bounds.GetExtent());
	BoundsOut.SphereRadius = FMath::Sqrt( RadiusSqr );
}


/**
 * Compute bounding box and sphere from vertices
 */
static void ComputeBoundsFromVertexList(const TArray<FStaticMeshBuildVertex>& Vertices, FBoxSphereBounds& BoundsOut)
{
	// Calculate the bounding box.
	FBounds3f Bounds;
	for (int32 VertexIndex = 0; VertexIndex < Vertices.Num(); VertexIndex++)
	{
		Bounds += Vertices[VertexIndex].Position;
	}
	
	// Calculate the bounding sphere, using the center of the bounding box as the origin.
	FVector3f Center = Bounds.GetCenter();
	float RadiusSqr = 0.0f;
	for (int32 VertexIndex = 0; VertexIndex < Vertices.Num(); VertexIndex++)
	{
		RadiusSqr = FMath::Max(	RadiusSqr, ( Vertices[VertexIndex].Position - Center ).SizeSquared() );
	}

	BoundsOut.Origin = FVector(Center);
	BoundsOut.BoxExtent = FVector(Bounds.GetExtent());
	BoundsOut.SphereRadius = FMath::Sqrt( RadiusSqr );
}

static void CorrectFallbackSettings( FMeshNaniteSettings& NaniteSettings, int32 NumTris )
{
	static const auto CVarFallbackThreshold = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("r.Nanite.Builder.FallbackTriangleThreshold"));

	switch( NaniteSettings.FallbackTarget )
	{
	case ENaniteFallbackTarget::Auto:
		NaniteSettings.FallbackPercentTriangles = 1.0f;
		NaniteSettings.FallbackRelativeError = NumTris <= CVarFallbackThreshold->GetValueOnAnyThread() ? 0.0f : 1.0f;
		break;
	case ENaniteFallbackTarget::PercentTriangles:
		NaniteSettings.FallbackRelativeError = 0.0f;
		break;
	case ENaniteFallbackTarget::RelativeError:
		NaniteSettings.FallbackPercentTriangles = 1.0f;
		break;
	}
}

struct FStaticMeshNaniteBuildContext
{
	FMeshNaniteSettings Settings;
	UStaticMesh* StaticMesh						= nullptr;
	const ITargetPlatform* TargetPlatform		= nullptr;
	const FStaticMeshSourceModel* SourceModel	= nullptr;
	Nanite::IBuilderModule* Builder 			= nullptr;

	bool bHiResSourceModel 		: 1	= false;

	bool IsValid() const { return StaticMesh != nullptr; }
};

static bool PrepareNaniteStaticMeshBuild(
	FStaticMeshNaniteBuildContext& OutContext,
	UStaticMesh* StaticMesh,
	const ITargetPlatform* TargetPlatform)
{
	if (!StaticMesh->IsNaniteEnabled())
	{
		// We don't need to build Nanite for this static mesh
		return false;
	}

	const bool bTargetSupportsNanite = DoesTargetPlatformSupportNanite(TargetPlatform);
	FStaticMeshSourceModel& LOD0SourceModel = StaticMesh->GetSourceModel(0);
	FStaticMeshSourceModel& HiResSourceModel = StaticMesh->GetHiResSourceModel();

	const FMeshDescription* LOD0MeshDescription = LOD0SourceModel.GetOrCacheMeshDescription();
	if (LOD0MeshDescription == nullptr)
	{
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Invalid mesh description during Nanite build [%s]."), *StaticMesh->GetFullName());
		return false;
	}
	if (LOD0MeshDescription->IsEmpty())
	{
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Empty mesh description during Nanite build [%s]."), *StaticMesh->GetFullName());
		return false;
	}

	// Only do Nanite build for the hi-res source model if we have one, the target platform supports Nanite, AND the mesh description
	// is well-formed. In all other cases, we will build Nanite from LOD0. This will replace the output VertexBuffers/etc with
	// the fractional Nanite cut to be stored as LOD0 RenderData.
	// NOTE: We also want to use LOD0 for targets that do not support Nanite (even if a hi-res source model was provided)
	// so that it generates the fallback, in which case the Nanite bulk will be stripped
	bool bUseHiResSourceModel = false;
	if (bTargetSupportsNanite && HiResSourceModel.IsMeshDescriptionValid())
	{
		if (const FMeshDescription* HiResMeshDescription = HiResSourceModel.GetOrCacheMeshDescription())
		{
			if (HiResMeshDescription->IsEmpty())
			{
				UE_LOG(LogStaticMeshBuilder, Display,
					TEXT("Invalid hi-res mesh description during Nanite build [%s]. The hi-res mesh is empty. ")
					TEXT("This is not supported and LOD 0 will be used as a fallback to build nanite data."),
					*StaticMesh->GetFullName());
			}
			else
			{
				// Make sure hi-res mesh data has the same amount of sections. If not, rendering bugs and issues will show
				// up because the nanite render must use the LOD 0 sections.
				if (HiResMeshDescription->PolygonGroups().Num() > LOD0MeshDescription->PolygonGroups().Num())
				{
					UE_LOG(LogStaticMeshBuilder, Display,
						TEXT("Invalid hi-res mesh description during Nanite build [%s]. ")
						TEXT("The number of sections from the hires mesh is higher than LOD 0 section count. ")
						TEXT("This is not supported and LOD 0 will be used as a fallback to build nanite data."),
						*StaticMesh->GetFullName());
				}
				else
				{
					if (HiResMeshDescription->PolygonGroups().Num() < LOD0MeshDescription->PolygonGroups().Num())
					{
						UE_LOG(LogStaticMeshBuilder, Display,
							TEXT("Nanite hi-res mesh description for [%s] has fewer sections than lod 0. ")
							TEXT("Verify you have the proper material id result when nanite is turned on."),
							*StaticMesh->GetFullName());
					}
					bUseHiResSourceModel = true;
				}
			}
		}
	}
	
	OutContext.Settings = StaticMesh->NaniteSettings;
	CorrectFallbackSettings(OutContext.Settings, LOD0MeshDescription->Triangles().Num());

	OutContext.StaticMesh 				= StaticMesh;
	OutContext.SourceModel				= bUseHiResSourceModel ? &HiResSourceModel : &LOD0SourceModel;
	OutContext.TargetPlatform			= TargetPlatform;
	OutContext.Builder					= &Nanite::IBuilderModule::Get();
	OutContext.bHiResSourceModel		= bUseHiResSourceModel;

	return true;
}

static bool InitNaniteBuildInput(
	FStaticMeshNaniteBuildContext& Context,
	Nanite::IBuilderModule::FInputMeshData& OutData,
	FBoxSphereBounds& OutBounds,
	bool& bOutNeeds32BitIndices)
{
	FMeshDescription MeshDescription; 
	if (!Context.SourceModel->CloneMeshDescription(MeshDescription))
	{
		UE_LOG(LogStaticMeshBuilder, Error,
			TEXT("Failed to clone mesh description during Nanite build [%s]."),
			*Context.StaticMesh->GetFullName());
		return false;
	}

	if (MeshDescription.IsEmpty())
	{
		UE_LOG(LogStaticMeshBuilder, Error,
			TEXT("Cannot build an empty mesh description during Nanite build [%s]."),
			*Context.StaticMesh->GetFullName());
		return false;
	}

	FMeshBuildSettings& BuildSettings = Context.StaticMesh->GetSourceModel(0).BuildSettings;

	// Only build tangents if they are explicitly enabled or we're going to be injecting this vertex data directly into
	// LOD0 of a generated fallback
	const bool bFallbackUsesInputMeshData =
		!Context.bHiResSourceModel &&
		Context.Settings.FallbackPercentTriangles == 1.0f &&
		Context.Settings.FallbackRelativeError == 0.0f;
	const bool bNeedTangents = Context.Settings.bExplicitTangents || bFallbackUsesInputMeshData;

	// compute tangents, lightmap UVs, etc
	FMeshDescriptionHelper MeshDescriptionHelper(&BuildSettings);
	MeshDescriptionHelper.SetupRenderMeshDescription(Context.StaticMesh, MeshDescription, true, bNeedTangents);

	// Prepare the PerSectionIndices array so we can optimize the index buffer for the GPU
	TArray<TArray<uint32>> PerSectionIndices;
	PerSectionIndices.AddDefaulted(MeshDescription.PolygonGroups().Num());
	OutData.Sections.Empty(MeshDescription.PolygonGroups().Num());

	// We only need this to de-duplicate vertices inside of BuildVertexBuffer
	// (And only if there are overlapping corners in the mesh description).
	TArray<int32> RemapVerts;

	// Nanite does not need the wedge map returned (mainly used by non-Nanite mesh painting).
	const bool bNeedWedgeMap = false;
	TArray<int32> WedgeMap;

	// Build the vertex and index buffer
	UE::Private::StaticMeshBuilder::BuildVertexBuffer(
		Context.StaticMesh,
		MeshDescription,
		BuildSettings,
		WedgeMap,
		OutData.Sections,
		PerSectionIndices,
		OutData.Vertices,
		MeshDescriptionHelper.GetOverlappingCorners(),
		RemapVerts,
		OutBounds,
		bNeedTangents,
		bNeedWedgeMap);

	// Concatenate the per-section index buffers.
	bOutNeeds32BitIndices = false;
	UE::Private::StaticMeshBuilder::BuildCombinedSectionIndices(
		PerSectionIndices,
		OutData.Sections,
		OutData.TriangleIndices,
		bOutNeeds32BitIndices);

	// Nanite build requires the section material indices to have already been resolved from the SectionInfoMap
	// as the indices are baked into the FMaterialTriangles.
	for (int32 SectionIndex = 0; SectionIndex < OutData.Sections.Num(); SectionIndex++)
	{
		OutData.Sections[SectionIndex].MaterialIndex = Context.StaticMesh->GetSectionInfoMap().Get(0, SectionIndex).MaterialIndex;
	}

	OutData.VertexBounds.Min = FVector4f(FVector3f(OutBounds.Origin - OutBounds.BoxExtent), 0.0f);
	OutData.VertexBounds.Max = FVector4f(FVector3f(OutBounds.Origin + OutBounds.BoxExtent), 0.0f);

	TVertexInstanceAttributesRef<FVector2f const> VertexInstanceUVs = MeshDescription.VertexInstanceAttributes().GetAttributesRef<FVector2f>(MeshAttribute::VertexInstance::TextureCoordinate);
	OutData.NumTexCoords = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;

	const uint32 TriangleCount = OutData.TriangleIndices.Num() / 3;
	OutData.TriangleCounts.Add(TriangleCount);

	if (!Context.Builder->BuildMaterialIndices(OutData.Sections, TriangleCount, OutData.MaterialIndices))
	{
		UE_LOGFMT_NSLOC(LogStaticMesh, Warning, "StaticMesh", "NaniteBuildError", "Failed to build Nanite from static mesh. See previous line(s) for details.");
		return false;
	}

	return true;
}

static void BuildNaniteFallbackMeshDescription(
	FStaticMeshNaniteBuildContext& Context,
	const Nanite::IBuilderModule::FOutputMeshData& InMeshData,
	FMeshDescription& OutMesh
)
{
	OutMesh.Empty();

	FStaticMeshAttributes Attributes(OutMesh);
	Attributes.Register();

	const int32 NumVertices = InMeshData.Vertices.Position.Num();
	const int32 NumUVChannels = InMeshData.Vertices.UVs.Num();
	const int32 NumTriangles = InMeshData.TriangleIndices.Num() / 3;
	const int32 NumPolyGroups = InMeshData.Sections.Num();

	OutMesh.ReserveNewVertices(NumVertices);
	OutMesh.ReserveNewVertexInstances(NumVertices);
	OutMesh.ReserveNewTriangles(NumTriangles);
	OutMesh.ReserveNewPolygonGroups(NumPolyGroups);

	OutMesh.SetNumUVChannels(NumUVChannels);
	OutMesh.VertexInstanceAttributes().SetAttributeChannelCount(MeshAttribute::VertexInstance::TextureCoordinate, NumUVChannels);
	for (int32 UVChannelIndex = 0; UVChannelIndex < NumUVChannels; ++UVChannelIndex)
	{
		OutMesh.ReserveNewUVs(NumVertices, UVChannelIndex);
	}

	TVertexAttributesRef<FVector3f> VertexPositions = Attributes.GetVertexPositions();
	TVertexInstanceAttributesRef<FVector3f> VertexInstanceNormals = Attributes.GetVertexInstanceNormals();
	TVertexInstanceAttributesRef<FVector3f> VertexInstanceTangents = Attributes.GetVertexInstanceTangents();
	TVertexInstanceAttributesRef<float> VertexInstanceBinormalSigns = Attributes.GetVertexInstanceBinormalSigns();
	TVertexInstanceAttributesRef<FVector4f> VertexInstanceColors = Attributes.GetVertexInstanceColors();
	TVertexInstanceAttributesRef<FVector2f> VertexInstanceUVs = Attributes.GetVertexInstanceUVs();
	TPolygonGroupAttributesRef<FName> PolygonGroupMaterialSlotNames = Attributes.GetPolygonGroupMaterialSlotNames();

	for (int32 InVertIndex = 0; InVertIndex < NumVertices; ++InVertIndex)
	{
		const FVertexID VertexID(InVertIndex);
		const FVertexInstanceID VertexInstanceID(InVertIndex);

		// TODO: Deduplicate vertex positions?
		OutMesh.CreateVertexWithID(VertexID);
		OutMesh.CreateVertexInstanceWithID(VertexInstanceID, VertexID);

		const FVector3f Position = InMeshData.Vertices.Position[InVertIndex];
		const FVector3f TangentX = InMeshData.Vertices.TangentX[InVertIndex];
		const FVector3f TangentY = InMeshData.Vertices.TangentY[InVertIndex];
		const FVector3f TangentZ = InMeshData.Vertices.TangentZ[InVertIndex];
		const float BinormalSign = GetBasisDeterminantSign(FVector(TangentX), FVector(TangentY), FVector(TangentZ));
		const FColor Color = InMeshData.Vertices.Color.IsValidIndex(InVertIndex) ?
			InMeshData.Vertices.Color[InVertIndex] : FColor::White;

		VertexPositions.Set(VertexID, InMeshData.Vertices.Position[InVertIndex]);
		VertexInstanceNormals.Set(VertexInstanceID, TangentZ);
		VertexInstanceTangents.Set(VertexInstanceID, TangentX);
		VertexInstanceBinormalSigns.Set(VertexInstanceID, BinormalSign);
		VertexInstanceColors.Set(VertexInstanceID, FVector4f(FLinearColor(Color)));

		for (int32 UVChannelIndex = 0; UVChannelIndex < NumUVChannels; ++UVChannelIndex)
		{
			const FVector2f UV = InMeshData.Vertices.UVs[UVChannelIndex][InVertIndex];
			VertexInstanceUVs.Set(VertexInstanceID, UVChannelIndex, UV);
		}
	}

	const TArray<FStaticMaterial>& StaticMaterials = Context.StaticMesh->GetStaticMaterials();
	for (const FStaticMeshSection& Section : InMeshData.Sections)
	{
		const FPolygonGroupID PolygonGroupID = OutMesh.CreatePolygonGroup();
		const FName MaterialSlotName = StaticMaterials.IsValidIndex(Section.MaterialIndex) ?
			StaticMaterials[Section.MaterialIndex].ImportedMaterialSlotName : NAME_None;
		PolygonGroupMaterialSlotNames.Set(PolygonGroupID, MaterialSlotName);

		for (uint32 TriIndex = 0; TriIndex < Section.NumTriangles; ++TriIndex)
		{
			const FVertexInstanceID TriVertInstanceIDs[] = {
				FVertexInstanceID(InMeshData.TriangleIndices[Section.FirstIndex + TriIndex * 3 + 0]),
				FVertexInstanceID(InMeshData.TriangleIndices[Section.FirstIndex + TriIndex * 3 + 1]),
				FVertexInstanceID(InMeshData.TriangleIndices[Section.FirstIndex + TriIndex * 3 + 2])
			};

			OutMesh.CreateTriangle(PolygonGroupID, MakeConstArrayView(TriVertInstanceIDs, 3));
		}
	}
}

static bool BuildNanite(
	FStaticMeshNaniteBuildContext& Context,
	FStaticMeshLODResources& LOD0Resources,
	FMeshDescription& LOD0MeshDescription,
	Nanite::FResources& NaniteResources,
	FBoxSphereBounds& BoundsOut
)
{
	if (!ensure(Context.IsValid()))
	{
		return false;
	}

	TRACE_CPUPROFILER_EVENT_SCOPE( FStaticMeshBuilder::BuildNanite );
	
	// Build new vertex buffers
	bool bNeeds32BitIndices;
	Nanite::IBuilderModule::FInputMeshData InputMeshData;
	if (!InitNaniteBuildInput(Context, InputMeshData, BoundsOut, bNeeds32BitIndices))
	{
		return false;
	}

	// Free up what we can from the input data as soon as the builder tells us it's done with it
	auto OnFreeInputMeshData = Nanite::IBuilderModule::FOnFreeInputMeshData::CreateLambda([&InputMeshData](bool bFallbackIsReduced)
	{
		if (bFallbackIsReduced)
		{
			InputMeshData.Vertices.Empty();
			InputMeshData.TriangleIndices.Empty();
		}

		InputMeshData.MaterialIndices.Empty();
	});

	// We don't need to generate a fallback when using a high res source model. Regular static mesh build will handle it
	const bool bGenerateFallback = !Context.bHiResSourceModel;
	Nanite::IBuilderModule::FOutputMeshData FallbackMeshData;
	
	if (!Context.Builder->Build(
			NaniteResources,
			InputMeshData,
			bGenerateFallback ? &FallbackMeshData : nullptr,
			Context.Settings,
			OnFreeInputMeshData
	))
	{
		UE_LOGFMT_NSLOC(LogStaticMesh, Warning, "StaticMesh", "NaniteHiResBuildError", "Failed to build Nanite for HiRes static mesh. See previous line(s) for details.");
		return false;
	}

	const FMeshBuildSettings& BuildSettings = Context.StaticMesh->GetSourceModel(0).BuildSettings;

	// Copy over the output data to the static mesh LOD data
	// Certain output LODs might be empty if the builder decided it wasn't needed (then remove these LODs again)
	// TODO: Is this ever the case with LOD 0 though?
	if (bGenerateFallback)
	{
		bool bHasValidSections = false;
		for (FStaticMeshSection& Section : FallbackMeshData.Sections)
		{
			if (Section.NumTriangles > 0)
			{
				bHasValidSections = true;
				break;
			}
		}

		// If there are valid sections then copy over data to the LODResource
		if (bHasValidSections)
		{
			LOD0Resources.Sections.Empty(FallbackMeshData.Sections.Num());
			for (FStaticMeshSection& Section : FallbackMeshData.Sections)
			{
				LOD0Resources.Sections.Add(Section);
			}

			TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build::BufferInit);

			FStaticMeshVertexBufferFlags StaticMeshVertexBufferFlags;
			StaticMeshVertexBufferFlags.bNeedsCPUAccess = true;
			StaticMeshVertexBufferFlags.bUseBackwardsCompatibleF16TruncUVs = BuildSettings.bUseBackwardsCompatibleF16TruncUVs;

			const FConstMeshBuildVertexView OutputMeshVertices = MakeConstMeshBuildVertexView(FallbackMeshData.Vertices);
			LOD0Resources.VertexBuffers.StaticMeshVertexBuffer.SetUseHighPrecisionTangentBasis(BuildSettings.bUseHighPrecisionTangentBasis);
			LOD0Resources.VertexBuffers.StaticMeshVertexBuffer.SetUseFullPrecisionUVs(BuildSettings.bUseFullPrecisionUVs);
			LOD0Resources.VertexBuffers.StaticMeshVertexBuffer.Init(OutputMeshVertices, StaticMeshVertexBufferFlags);
			LOD0Resources.VertexBuffers.PositionVertexBuffer.Init(OutputMeshVertices);
			LOD0Resources.VertexBuffers.ColorVertexBuffer.Init(OutputMeshVertices);

			// Why is the 'bNeeds32BitIndices' used from the original index buffer? Is that needed?
			const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;
			LOD0Resources.IndexBuffer.SetIndices(FallbackMeshData.TriangleIndices, IndexBufferStride);

			BuildAllBufferOptimizations(LOD0Resources, BuildSettings, FallbackMeshData.TriangleIndices, bNeeds32BitIndices, OutputMeshVertices);

			// Fill out the mesh description for non-Nanite build/reduction
			BuildNaniteFallbackMeshDescription(Context, FallbackMeshData, LOD0MeshDescription);
		}
		else
		{
			// Initialize the mesh description as empty
			FStaticMeshAttributes(LOD0MeshDescription).Register();
		}
	}

	return true;
}


bool FStaticMeshBuilder::Build(FStaticMeshRenderData& StaticMeshRenderData, const FStaticMeshBuildParameters& BuildParameters)
{
	if (BuildParameters.TargetPlatform == nullptr)
	{
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Provided FStaticMeshBuildParameters must have a valid TargetPlatform."));
		return false;
	}

	UStaticMesh* StaticMesh = BuildParameters.StaticMesh;
	const FStaticMeshLODGroup& LODGroup = BuildParameters.LODGroup;

	if (!StaticMesh->IsMeshDescriptionValid(0))
	{
		//Warn the user that there is no mesh description data
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot find a valid mesh description to build the asset."));
		return false;
	}

	if (StaticMeshRenderData.LODResources.Num() > 0)
	{
		//At this point the render data is suppose to be empty
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot build static mesh render data twice [%s]."), *StaticMesh->GetFullName());
		
		//Crash in debug
		checkSlow(StaticMeshRenderData.LODResources.Num() == 0);

		return false;
	}

	TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build);

	const int32 NumSourceModels = StaticMesh->GetNumSourceModels();
	StaticMeshRenderData.AllocateLODResources(NumSourceModels);

	FStaticMeshNaniteBuildContext NaniteBuildContext;
	const bool bBuildNanite = PrepareNaniteStaticMeshBuild(NaniteBuildContext, StaticMesh, BuildParameters.TargetPlatform);

	const int32 NumTasks = NaniteBuildContext.bHiResSourceModel ? (NumSourceModels + 1) : NumSourceModels;
	FScopedSlowTask SlowTask(NumTasks, NSLOCTEXT("StaticMeshEditor", "StaticMeshBuilderBuild", "Building static mesh render data."));
	SlowTask.MakeDialog();

	FBoxSphereBounds::Builder MeshBoundsBuilder;

	const FMeshSectionInfoMap BeforeBuildSectionInfoMap = StaticMesh->GetSectionInfoMap();
	const FMeshSectionInfoMap BeforeBuildOriginalSectionInfoMap = StaticMesh->GetOriginalSectionInfoMap();

	TArray<FMeshDescription> MeshDescriptions;
	MeshDescriptions.SetNum(NumSourceModels);

	int32 NaniteBuiltLevels = 0;

	if (bBuildNanite)
	{
		SlowTask.EnterProgressFrame( 1 );

		Nanite::FResources& NaniteResources = *StaticMeshRenderData.NaniteResourcesPtr.Get();
		FBoxSphereBounds NaniteBounds;
		bool bBuildSuccess = BuildNanite(
			NaniteBuildContext,
			StaticMeshRenderData.LODResources[0],
			MeshDescriptions[0],
			NaniteResources,
			NaniteBounds);

		if (bBuildSuccess)
		{
			MeshBoundsBuilder += NaniteBounds;
			if (!NaniteBuildContext.bHiResSourceModel)
			{
				// We don't need to build LOD 0 below if the Nanite build generated it
				++NaniteBuiltLevels;
			}
		}
	}

	// Build non-Nanite render data for each LOD
	for (int32 LodIndex = NaniteBuiltLevels; LodIndex < NumSourceModels; ++LodIndex)
	{
		TRACE_CPUPROFILER_EVENT_SCOPE_STR("FStaticMeshBuilder::Build LOD");
		SlowTask.EnterProgressFrame(1);
		FScopedSlowTask BuildLODSlowTask(3);
		BuildLODSlowTask.EnterProgressFrame(1);

		FStaticMeshSourceModel& SrcModel = StaticMesh->GetSourceModel(LodIndex);

		// NOTE: Make a local copy on the stack, as build settings are used to generate the DDC key for static mesh, and
		// the mesh description helper might make changes to validate some settings
		FMeshBuildSettings LODBuildSettings = SrcModel.BuildSettings;

		float MaxDeviation = 0.0f;
		bool bIsMeshDescriptionValid = StaticMesh->CloneMeshDescription(LodIndex, MeshDescriptions[LodIndex]);
		bIsMeshDescriptionValid &= !MeshDescriptions[LodIndex].IsEmpty();
		FMeshDescriptionHelper MeshDescriptionHelper(&LODBuildSettings);

		FMeshReductionSettings ReductionSettings = LODGroup.GetSettings(SrcModel.ReductionSettings, LodIndex);

		// Make sure we do not reduce a non custom LOD by itself
		const int32 BaseReduceLodIndex = FMath::Clamp<int32>(ReductionSettings.BaseLODModel, 0, bIsMeshDescriptionValid ? LodIndex : LodIndex - 1);
		// Use simplifier if a reduction in triangles or verts has been requested.
		bool bUseReduction = StaticMesh->IsReductionActive(LodIndex);

		if (bIsMeshDescriptionValid)
		{
			MeshDescriptionHelper.SetupRenderMeshDescription(StaticMesh, MeshDescriptions[LodIndex], false, true);
			//Make sure the cache is good before looking for the active reduction
			if (SrcModel.CacheMeshDescriptionTrianglesCount == MAX_uint32)
			{
				SrcModel.CacheMeshDescriptionTrianglesCount = static_cast<uint32>(MeshDescriptions[LodIndex].Triangles().Num());
			}
			if (SrcModel.CacheMeshDescriptionVerticesCount == MAX_uint32)
			{
				SrcModel.CacheMeshDescriptionVerticesCount = static_cast<uint32>(FStaticMeshOperations::GetUniqueVertexCount(MeshDescriptions[LodIndex], MeshDescriptionHelper.GetOverlappingCorners()));
			}
			//Get back the reduction status once we apply all build settings, vertex count can change depending on the build settings
			bUseReduction = StaticMesh->IsReductionActive(LodIndex);
		}
		else
		{
			if (bUseReduction)
			{
				// Initialize an empty mesh description that the reduce will fill
				FStaticMeshAttributes(MeshDescriptions[LodIndex]).Register();
			}
			else
			{
				//Duplicate the lodindex 0 we have a 100% reduction which is like a duplicate
				MeshDescriptions[LodIndex] = MeshDescriptions[BaseReduceLodIndex];
				//Set the overlapping threshold
				float ComparisonThreshold = StaticMesh->GetSourceModel(BaseReduceLodIndex).BuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;
				MeshDescriptionHelper.FindOverlappingCorners(MeshDescriptions[LodIndex], ComparisonThreshold);
				if (LodIndex > 0)
				{
					
					//Make sure the SectionInfoMap is taken from the Base RawMesh
					int32 SectionNumber = StaticMesh->GetOriginalSectionInfoMap().GetSectionNumber(BaseReduceLodIndex);
					for (int32 SectionIndex = 0; SectionIndex < SectionNumber; ++SectionIndex)
					{
						//Keep the old data if its valid
						bool bHasValidLODInfoMap = StaticMesh->GetSectionInfoMap().IsValidSection(LodIndex, SectionIndex);
						//Section material index have to be remap with the ReductionSettings.BaseLODModel SectionInfoMap to create
						//a valid new section info map for the reduced LOD.
						if (!bHasValidLODInfoMap && StaticMesh->GetSectionInfoMap().IsValidSection(BaseReduceLodIndex, SectionIndex))
						{
							//Copy the BaseLODModel section info to the reduce LODIndex.
							FMeshSectionInfo SectionInfo = StaticMesh->GetSectionInfoMap().Get(BaseReduceLodIndex, SectionIndex);
							FMeshSectionInfo OriginalSectionInfo = StaticMesh->GetOriginalSectionInfoMap().Get(BaseReduceLodIndex, SectionIndex);
							StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
							StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, SectionIndex, OriginalSectionInfo);
						}
					}
				}
			}

			if (LodIndex > 0)
			{
				LODBuildSettings = StaticMesh->GetSourceModel(BaseReduceLodIndex).BuildSettings;
			}
		}

		// Reduce LODs
		if (bUseReduction)
		{
			TRACE_CPUPROFILER_EVENT_SCOPE_STR("FStaticMeshBuilder::Build - Reduce LOD");
			
			float OverlappingThreshold = LODBuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;
			FOverlappingCorners OverlappingCorners;
			FStaticMeshOperations::FindOverlappingCorners(OverlappingCorners, MeshDescriptions[BaseReduceLodIndex], OverlappingThreshold);

			int32 OldSectionInfoMapCount = StaticMesh->GetSectionInfoMap().GetSectionNumber(LodIndex);

			TFunction<void(const FMeshDescription&, const FMeshDescription&)> CheckReduction = [&](const FMeshDescription& InitMesh, const FMeshDescription& ReducedMesh)
			{
				FBox BBoxInitMesh = InitMesh.ComputeBoundingBox();
				double BBoxInitMeshSize = (BBoxInitMesh.Max - BBoxInitMesh.Min).Length();

				FBox BBoxReducedMesh = ReducedMesh.ComputeBoundingBox();
				double BBoxReducedMeshSize = (BBoxReducedMesh.Max - BBoxReducedMesh.Min).Length();

				constexpr double ThresholdForAbnormalGrowthOfBBox = UE_DOUBLE_SQRT_3; // the reduced mesh must stay in the bounding sphere 
				if (BBoxReducedMeshSize > BBoxInitMeshSize * ThresholdForAbnormalGrowthOfBBox)
				{
					UE_LOG(LogStaticMeshBuilder, Warning, TEXT("The generation of LOD could have generated spikes on the mesh for %s"), *StaticMesh->GetName());
				}
			};

			if (LodIndex == BaseReduceLodIndex)
			{
				//When using LOD 0, we use a copy of the mesh description since reduce do not support inline reducing
				FMeshDescription BaseMeshDescription = MeshDescriptions[BaseReduceLodIndex];
				MeshDescriptionHelper.ReduceLOD(BaseMeshDescription, MeshDescriptions[LodIndex], ReductionSettings, OverlappingCorners, MaxDeviation);
				CheckReduction(BaseMeshDescription, MeshDescriptions[LodIndex]);
			}
			else
			{
				MeshDescriptionHelper.ReduceLOD(MeshDescriptions[BaseReduceLodIndex], MeshDescriptions[LodIndex], ReductionSettings, OverlappingCorners, MaxDeviation);
				CheckReduction(MeshDescriptions[BaseReduceLodIndex], MeshDescriptions[LodIndex]);
			}

			const TPolygonGroupAttributesRef<FName> PolygonGroupImportedMaterialSlotNames = MeshDescriptions[LodIndex].PolygonGroupAttributes().GetAttributesRef<FName>(MeshAttribute::PolygonGroup::ImportedMaterialSlotName);
			const TPolygonGroupAttributesRef<FName> BasePolygonGroupImportedMaterialSlotNames = MeshDescriptions[BaseReduceLodIndex].PolygonGroupAttributes().GetAttributesRef<FName>(MeshAttribute::PolygonGroup::ImportedMaterialSlotName);
			// Recompute adjacency information. Since we change the vertices when we reduce
			MeshDescriptionHelper.FindOverlappingCorners(MeshDescriptions[LodIndex], OverlappingThreshold);
			
			//Make sure the static mesh SectionInfoMap is up to date with the new reduce LOD
			//We have to remap the material index with the ReductionSettings.BaseLODModel sectionInfoMap
			//Set the new SectionInfoMap for this reduced LOD base on the ReductionSettings.BaseLODModel SectionInfoMap
			TArray<int32> BaseUniqueMaterialIndexes;
			//Find all unique Material in used order
			for (const FPolygonGroupID PolygonGroupID : MeshDescriptions[BaseReduceLodIndex].PolygonGroups().GetElementIDs())
			{
				int32 MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(BasePolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
				if (MaterialIndex == INDEX_NONE)
				{
					MaterialIndex = PolygonGroupID.GetValue();
				}
				BaseUniqueMaterialIndexes.AddUnique(MaterialIndex);
			}
			TArray<int32> UniqueMaterialIndex;
			//Find all unique Material in used order
			for (const FPolygonGroupID PolygonGroupID : MeshDescriptions[LodIndex].PolygonGroups().GetElementIDs())
			{
				int32 MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(PolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
				if (MaterialIndex == INDEX_NONE)
				{
					MaterialIndex = PolygonGroupID.GetValue();
				}
				UniqueMaterialIndex.AddUnique(MaterialIndex);
			}

			//If the reduce did not output the same number of section use the base LOD sectionInfoMap
			bool bIsOldMappingInvalid = OldSectionInfoMapCount != MeshDescriptions[LodIndex].PolygonGroups().Num();

			bool bValidBaseSectionInfoMap = BeforeBuildSectionInfoMap.GetSectionNumber(BaseReduceLodIndex) > 0;
			//All used material represent a different section
			for (int32 SectionIndex = 0; SectionIndex < UniqueMaterialIndex.Num(); ++SectionIndex)
			{
				//Keep the old data
				bool bHasValidLODInfoMap = !bIsOldMappingInvalid && BeforeBuildSectionInfoMap.IsValidSection(LodIndex, SectionIndex);
				//Section material index have to be remap with the ReductionSettings.BaseLODModel SectionInfoMap to create
				//a valid new section info map for the reduced LOD.

				//Find the base LOD section using this material
				if (!bHasValidLODInfoMap)
				{
					bool bSectionInfoSet = false;
					if (bValidBaseSectionInfoMap)
					{
						for (int32 BaseSectionIndex = 0; BaseSectionIndex < BaseUniqueMaterialIndexes.Num(); ++BaseSectionIndex)
						{
							if (UniqueMaterialIndex[SectionIndex] == BaseUniqueMaterialIndexes[BaseSectionIndex])
							{
								//Copy the base sectionInfoMap
								FMeshSectionInfo SectionInfo = BeforeBuildSectionInfoMap.Get(BaseReduceLodIndex, BaseSectionIndex);
								FMeshSectionInfo OriginalSectionInfo = BeforeBuildOriginalSectionInfoMap.Get(BaseReduceLodIndex, BaseSectionIndex);
								StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
								StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, BaseSectionIndex, OriginalSectionInfo);
								bSectionInfoSet = true;
								break;
							}
						}
					}

					if (!bSectionInfoSet)
					{
						//Just set the default section info in case we did not found any match with the Base Lod
						FMeshSectionInfo SectionInfo;
						SectionInfo.MaterialIndex = SectionIndex;
						StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
						StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
					}
				}
			}
		}
		BuildLODSlowTask.EnterProgressFrame(1);
		const FPolygonGroupArray& PolygonGroups = MeshDescriptions[LodIndex].PolygonGroups();

		FStaticMeshLODResources& StaticMeshLOD = StaticMeshRenderData.LODResources[LodIndex];
		StaticMeshLOD.MaxDeviation = MaxDeviation;

		// Build new vertex buffers
		FMeshBuildVertexData BuildVertexData;

		StaticMeshLOD.Sections.Empty(PolygonGroups.Num());
		TArray<int32> RemapVerts; //Because we will remove MeshVertex that are redundant, we need a remap
									//Render data Wedge map is only set for LOD 0???

		TArray<int32>& WedgeMap = StaticMeshLOD.WedgeMap;
		WedgeMap.Reset();

		// Prepare the PerSectionIndices array so we can optimize the index buffer for the GPU
		TArray<TArray<uint32> > PerSectionIndices;
		PerSectionIndices.AddDefaulted(MeshDescriptions[LodIndex].PolygonGroups().Num());

		FBoxSphereBounds LODBounds;

		// Build the vertex and index buffer
		UE::Private::StaticMeshBuilder::BuildVertexBuffer(
			StaticMesh,
			MeshDescriptions[LodIndex],
			LODBuildSettings,
			WedgeMap,
			StaticMeshLOD.Sections,
			PerSectionIndices,
			BuildVertexData,
			MeshDescriptionHelper.GetOverlappingCorners(),
			RemapVerts,
			LODBounds,
			true /* bNeedTangents */,
			true /* bNeedWedgeMap */
		);

		MeshBoundsBuilder += LODBounds;

		TVertexInstanceAttributesRef<FVector2f> VertexInstanceUVs = MeshDescriptions[LodIndex].VertexInstanceAttributes().GetAttributesRef<FVector2f>(MeshAttribute::VertexInstance::TextureCoordinate);
		const uint32 NumTextureCoord = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;

		// Only the render data and vertex buffers will be used from now on unless we have more than one source models
		// This will help with memory usage for Nanite Mesh by releasing memory before doing the build
		if (NumSourceModels == 1)
		{
			MeshDescriptions.Empty();
		}

		// Concatenate the per-section index buffers.
		TArray<uint32> CombinedIndices;
		bool bNeeds32BitIndices = false;
		UE::Private::StaticMeshBuilder::BuildCombinedSectionIndices(PerSectionIndices, StaticMeshLOD.Sections, CombinedIndices, bNeeds32BitIndices);

		{
			TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build::BufferInit);

			FConstMeshBuildVertexView ConstVertexView = MakeConstMeshBuildVertexView(BuildVertexData);

			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseHighPrecisionTangentBasis(LODBuildSettings.bUseHighPrecisionTangentBasis);
			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseFullPrecisionUVs(LODBuildSettings.bUseFullPrecisionUVs);
			FStaticMeshVertexBufferFlags StaticMeshVertexBufferFlags;
			StaticMeshVertexBufferFlags.bNeedsCPUAccess = true;
			StaticMeshVertexBufferFlags.bUseBackwardsCompatibleF16TruncUVs = LODBuildSettings.bUseBackwardsCompatibleF16TruncUVs;
			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.Init(ConstVertexView, StaticMeshVertexBufferFlags);
			StaticMeshLOD.VertexBuffers.PositionVertexBuffer.Init(ConstVertexView);
			StaticMeshLOD.VertexBuffers.ColorVertexBuffer.Init(ConstVertexView);

			const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;
			StaticMeshLOD.IndexBuffer.SetIndices(CombinedIndices, IndexBufferStride);

			// post-process the index buffer
			BuildLODSlowTask.EnterProgressFrame(1);
			BuildAllBufferOptimizations(StaticMeshLOD, LODBuildSettings, CombinedIndices, bNeeds32BitIndices, ConstVertexView);
		}

	} //End of LOD for loop

	// Update the render data bounds
	StaticMeshRenderData.Bounds = MeshBoundsBuilder;

	if (StaticMesh->bSupportRayTracing && BuildParameters.TargetPlatform->UsesRayTracing())
	{
		const bool bUsingRenderingLODs = true;

		if (bUsingRenderingLODs)
		{
			StaticMeshRenderData.InitializeRayTracingRepresentationFromRenderingLODs();
		}
		else
		{
			unimplemented();
		}
	}
	
	return true;
}

bool FStaticMeshBuilder::Build(
	FStaticMeshRenderData& OutRenderData,
	UStaticMesh* StaticMesh,
	const FStaticMeshLODGroup& LODGroup,
	bool bAllowNanite)
{
	return Build(OutRenderData, FStaticMeshBuildParameters(StaticMesh, nullptr, LODGroup));
}

bool FStaticMeshBuilder::BuildMeshVertexPositions(
	UStaticMesh* StaticMesh,
	TArray<uint32>& BuiltIndices,
	TArray<FVector3f>& BuiltVertices,
	FStaticMeshSectionArray& Sections)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::BuildMeshVertexPositions);

	FStaticMeshSourceModel& SourceModel = StaticMesh->IsHiResMeshDescriptionValid() ? StaticMesh->GetHiResSourceModel() : StaticMesh->GetSourceModel(0);
	if (!SourceModel.IsMeshDescriptionValid())
	{
		//Warn the user that there is no mesh description data
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot find a valid mesh description to build the asset."));
		return false;
	}

	FMeshDescription MeshDescription;
	const bool bIsMeshDescriptionValid = SourceModel.CloneMeshDescription(MeshDescription);
	check(bIsMeshDescriptionValid);

	if (MeshDescription.IsEmpty())
	{
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot build the asset from an empty mesh description."));
		return false;
	}

	FMeshBuildSettings& BuildSettings = StaticMesh->GetSourceModel(0).BuildSettings;

	FMeshDescriptionHelper MeshDescriptionHelper(&BuildSettings);
	MeshDescriptionHelper.SetupRenderMeshDescription(StaticMesh, MeshDescription, false, false);

	const FPolygonGroupArray& PolygonGroups = MeshDescription.PolygonGroups();

	// Build new vertex buffers
	FMeshBuildVertexData BuildVertexData;

	Sections.Empty(PolygonGroups.Num());

	TArray<int32> RemapVerts; //Because we will remove MeshVertex that are redundant, we need a remap
	//Render data Wedge map is only set for LOD 0???

	TArray<int32> WedgeMap;

	// Prepare the PerSectionIndices array so we can optimize the index buffer for the GPU
	TArray<TArray<uint32>> PerSectionIndices;
	PerSectionIndices.AddDefaulted(MeshDescription.PolygonGroups().Num());

	FBoxSphereBounds LODBounds;

	// Build the vertex and index buffer
	UE::Private::StaticMeshBuilder::BuildVertexBuffer(
		StaticMesh,
		MeshDescription,
		BuildSettings,
		WedgeMap,
		Sections,
		PerSectionIndices,
		BuildVertexData,
		MeshDescriptionHelper.GetOverlappingCorners(),
		RemapVerts,
		LODBounds,
		false /* bNeedTangents */,
		false /* bNeedWedgeMap */
	);

	BuiltVertices = BuildVertexData.Position;

	// Release MeshDescription memory since we don't need it anymore
	MeshDescription.Empty();

	// Concatenate the per-section index buffers.
	bool bNeeds32BitIndices = false;
	UE::Private::StaticMeshBuilder::BuildCombinedSectionIndices(PerSectionIndices, Sections, BuiltIndices, bNeeds32BitIndices);

	// Apply section remapping
	for (int32 SectionIndex = 0; SectionIndex < Sections.Num(); SectionIndex++)
	{
		Sections[SectionIndex].MaterialIndex = StaticMesh->GetSectionInfoMap().Get(0, SectionIndex).MaterialIndex;
	}

	return true;
}

namespace UE::Private::StaticMeshBuilder
{

struct FPendingVertex
{
	FVector3f Position;
	FVector3f TangentX;
	FVector3f TangentY;
	FVector3f TangentZ;
	FColor Color;
	TStaticArray<FVector2f, MAX_STATIC_TEXCOORDS> UVs;
};

bool AreVerticesEqual(const FPendingVertex& Vertex, const FMeshBuildVertexData& VertexData, int32 CompareVertex, float ComparisonThreshold)
{
	if (!Vertex.Position.Equals(VertexData.Position[CompareVertex], ComparisonThreshold))
	{
		return false;
	}

	// Test TangentZ first, often X and Y are zero
	if (!NormalsEqual(Vertex.TangentZ, VertexData.TangentZ[CompareVertex]))
	{
		return false;
	}

	if (!NormalsEqual(Vertex.TangentX, VertexData.TangentX[CompareVertex]))
	{
		return false;
	}

	if (!NormalsEqual(Vertex.TangentY, VertexData.TangentY[CompareVertex]))
	{
		return false;
	}

	if (VertexData.Color.Num() > 0)
	{
		if (Vertex.Color != VertexData.Color[CompareVertex])
		{
			return false;
		}
	}

	// UVs
	for (int32 UVIndex = 0; UVIndex < VertexData.UVs.Num(); UVIndex++)
	{
		if (!UVsEqual(Vertex.UVs[UVIndex], VertexData.UVs[UVIndex][CompareVertex]))
		{
			return false;
		}
	}

	return true;
}

void BuildVertexBuffer(
	UStaticMesh* StaticMesh,
	const FMeshDescription& MeshDescription,
	const FMeshBuildSettings& BuildSettings,
	TArray<int32>& OutWedgeMap,
	FStaticMeshSectionArray& OutSections,
	TArray<TArray<uint32>>& OutPerSectionIndices,
	FMeshBuildVertexData& BuildVertexData,
	const FOverlappingCorners& OverlappingCorners,
	TArray<int32>& RemapVerts,
	FBoxSphereBounds& MeshBounds,
	bool bNeedTangents,
	bool bNeedWedgeMap
)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(BuildVertexBuffer);

	const int32 NumVertexInstances = MeshDescription.VertexInstances().GetArraySize();
	const bool bCacheOptimize = (NumVertexInstances < 100000 * 3);

	FBounds3f Bounds;
	bool bBoundsSet = false;

	FStaticMeshConstAttributes Attributes(MeshDescription);

	TPolygonGroupAttributesConstRef<FName> PolygonGroupImportedMaterialSlotNames = Attributes.GetPolygonGroupMaterialSlotNames();
	TVertexAttributesConstRef<FVector3f> VertexPositions = Attributes.GetVertexPositions();
	TVertexInstanceAttributesConstRef<FVector3f> VertexInstanceNormals = Attributes.GetVertexInstanceNormals();
	TVertexInstanceAttributesConstRef<FVector3f> VertexInstanceTangents = Attributes.GetVertexInstanceTangents();
	TVertexInstanceAttributesConstRef<float> VertexInstanceBinormalSigns = Attributes.GetVertexInstanceBinormalSigns();
	TVertexInstanceAttributesConstRef<FVector4f> VertexInstanceColors = Attributes.GetVertexInstanceColors();
	TVertexInstanceAttributesConstRef<FVector2f> VertexInstanceUVs = Attributes.GetVertexInstanceUVs();

	const bool bHasColors = VertexInstanceColors.IsValid();
	bool bValidColors = false;
	const int32 NumTextureCoord = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;
	const FVector3f BuildScale(BuildSettings.BuildScale3D);

	// set up vertex buffer elements
	BuildVertexData.Position.Reserve(NumVertexInstances);
	BuildVertexData.TangentX.Reserve(NumVertexInstances);
	BuildVertexData.TangentY.Reserve(NumVertexInstances);
	BuildVertexData.TangentZ.Reserve(NumVertexInstances);
	BuildVertexData.UVs.SetNum(NumTextureCoord);
	for (int32 TexCoord = 0; TexCoord < NumTextureCoord; ++TexCoord)
	{
		BuildVertexData.UVs[TexCoord].Reserve(NumVertexInstances);
	}

	TMap<FPolygonGroupID, int32> PolygonGroupToSectionIndex;

	for (const FPolygonGroupID PolygonGroupID : MeshDescription.PolygonGroups().GetElementIDs())
	{
		int32& SectionIndex = PolygonGroupToSectionIndex.FindOrAdd(PolygonGroupID);
		SectionIndex = OutSections.Add(FStaticMeshSection());
		FStaticMeshSection& StaticMeshSection = OutSections[SectionIndex];
		StaticMeshSection.MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(PolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
		if (StaticMeshSection.MaterialIndex == INDEX_NONE)
		{
			StaticMeshSection.MaterialIndex = PolygonGroupID.GetValue();
		}
	}

	int32 ReserveIndicesCount = MeshDescription.Triangles().Num() * 3;

	// Fill the remap array
	{
		RemapVerts.AddZeroed(ReserveIndicesCount);
		for (int32& RemapIndex : RemapVerts)
		{
			RemapIndex = INDEX_NONE;
		}
	}

	// Initialize the wedge map array tracking correspondence between wedge index and rendering vertex index
	OutWedgeMap.Reset();
	if (bNeedWedgeMap)
	{
		OutWedgeMap.AddZeroed(ReserveIndicesCount);
	}

	float VertexComparisonThreshold = BuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;

	bool bUseLegacyTangentScaling = StaticMesh->GetLegacyTangentScaling();

	int32 WedgeIndex = 0;
	for (const FTriangleID TriangleID : MeshDescription.Triangles().GetElementIDs())
	{
		const FPolygonGroupID PolygonGroupID = MeshDescription.GetTrianglePolygonGroup(TriangleID);
		const int32 SectionIndex = PolygonGroupToSectionIndex[PolygonGroupID];
		TArray<uint32>& SectionIndices = OutPerSectionIndices[SectionIndex];

		TArrayView<const FVertexID> VertexIDs = MeshDescription.GetTriangleVertices(TriangleID);

		FVector3f CornerPositions[3];
		for (int32 TriVert = 0; TriVert < 3; ++TriVert)
		{
			CornerPositions[TriVert] = VertexPositions[VertexIDs[TriVert]];
		}
		FOverlappingThresholds OverlappingThresholds;
		OverlappingThresholds.ThresholdPosition = VertexComparisonThreshold;
		// Don't process degenerate triangles.
		if (PointsEqual(CornerPositions[0], CornerPositions[1], OverlappingThresholds)
			|| PointsEqual(CornerPositions[0], CornerPositions[2], OverlappingThresholds)
			|| PointsEqual(CornerPositions[1], CornerPositions[2], OverlappingThresholds))
		{
			WedgeIndex += 3;
			continue;
		}

		TArrayView<const FVertexInstanceID> VertexInstanceIDs = MeshDescription.GetTriangleVertexInstances(TriangleID);
		for (int32 TriVert = 0; TriVert < 3; ++TriVert, ++WedgeIndex)
		{
			const FVertexInstanceID VertexInstanceID = VertexInstanceIDs[TriVert];
			const FVector3f& VertexPosition = CornerPositions[TriVert];
			const FVector3f& VertexInstanceNormal = VertexInstanceNormals[VertexInstanceID];
			const FVector3f& VertexInstanceTangent = VertexInstanceTangents[VertexInstanceID];
			const float VertexInstanceBinormalSign = VertexInstanceBinormalSigns[VertexInstanceID];

			FPendingVertex PendingVertex;

			PendingVertex.Position = VertexPosition * BuildScale;
			if (bNeedTangents)
			{
				if (bUseLegacyTangentScaling)
				{
					// Apply incorrect inverse scale to tangents to match an old bug, for legacy assets only
					PendingVertex.TangentX = VertexInstanceTangent / BuildScale;
					PendingVertex.TangentY = ((VertexInstanceNormal ^ VertexInstanceTangent) * VertexInstanceBinormalSign) / BuildScale;
				}
				else
				{
					// Tangents should transform by directly applying the same scale as the geometry; it's only the normal that needs an inverse scale
					PendingVertex.TangentX = VertexInstanceTangent * BuildScale;
					PendingVertex.TangentY = ((VertexInstanceNormal ^ VertexInstanceTangent) * VertexInstanceBinormalSign) * BuildScale;
				}
				PendingVertex.TangentX.Normalize();
				PendingVertex.TangentY.Normalize();
			}
			else
			{
				PendingVertex.TangentX = FVector3f(1.0f, 0.0f, 0.0f);
				PendingVertex.TangentY = FVector3f(0.0f, 1.0f, 0.0f);
			}
			PendingVertex.TangentZ = VertexInstanceNormal / BuildScale;
			PendingVertex.TangentZ.Normalize();

			FColor VertexColor = FColor::White;
			if (bHasColors)
			{
				const FVector4f& VertexInstanceColor = VertexInstanceColors[VertexInstanceID];
				const FLinearColor LinearColor(VertexInstanceColor);
				VertexColor = LinearColor.ToFColor(true);
			}

			PendingVertex.Color = VertexColor;

			for (int32 UVIndex = 0; UVIndex < NumTextureCoord; ++UVIndex)
			{
				PendingVertex.UVs[UVIndex] = VertexInstanceUVs.Get(VertexInstanceID, UVIndex);
			}

			int32 Index = INDEX_NONE;

			// Never add duplicated vertex instance
			// Use WedgeIndex since OverlappingCorners has been built based on that
			{
				const TArray<int32>& DupVerts = OverlappingCorners.FindIfOverlapping(WedgeIndex);
				for (int32 k = 0; k < DupVerts.Num(); k++)
				{
					if (DupVerts[k] >= WedgeIndex)
					{
						break;
					}
					int32 Location = RemapVerts.IsValidIndex(DupVerts[k]) ? RemapVerts[DupVerts[k]] : INDEX_NONE;
					if (Location != INDEX_NONE && AreVerticesEqual(PendingVertex, BuildVertexData, Location, VertexComparisonThreshold))
					{
						Index = Location;
						break;
					}
				}
			}

			if (Index == INDEX_NONE)
			{
				Index = BuildVertexData.Position.Emplace(PendingVertex.Position);
				
				BuildVertexData.TangentX.Emplace(PendingVertex.TangentX);
				BuildVertexData.TangentY.Emplace(PendingVertex.TangentY);
				BuildVertexData.TangentZ.Emplace(PendingVertex.TangentZ);
				
				if (bHasColors)
				{
					if (PendingVertex.Color != FColor::White)
					{
						bValidColors = true;
					}

					if (BuildVertexData.Color.Num() == 0 && bValidColors)
					{
						// First occurrence of a non fully opaque white color means we allocate output space,
						// and then set all previously encountered vertex colors to be opaque white.
						BuildVertexData.Color.Reserve(NumVertexInstances);
						BuildVertexData.Color.SetNumUninitialized(BuildVertexData.Position.Num() - 1);
						for (int32 ColorIndex = 0; ColorIndex < BuildVertexData.Color.Num(); ++ColorIndex)
						{
							BuildVertexData.Color[ColorIndex] = FColor::White;
						}
					}

					if (bValidColors)
					{
						BuildVertexData.Color.Emplace(PendingVertex.Color);
					}
				}

				for (int32 UVIndex = 0; UVIndex < NumTextureCoord; ++UVIndex)
				{
					BuildVertexData.UVs[UVIndex].Emplace(VertexInstanceUVs.Get(VertexInstanceID, UVIndex));
				}

				// We are already processing all vertices, so we may as well compute the bounding box here
				// instead of yet another loop over the vertices at a later point.
				Bounds += PendingVertex.Position;
				bBoundsSet = true;
			}

				RemapVerts[WedgeIndex] = Index;

			if (bNeedWedgeMap)
			{
				OutWedgeMap[WedgeIndex] = Index;
			}

			SectionIndices.Add(Index);
		}
	}

	if (!bBoundsSet)
	{
		// There were no verts that contribute to bounds, so we'll just set a bounds of 0,0,0 to avoid calculating NaNs for Origin, BoxExtent, and SphereRadius below
		Bounds = FVector3f(0.f, 0.f, 0.f);
	}

	// Calculate the bounding sphere, using the center of the bounding box as the origin.
	FVector3f Center = Bounds.GetCenter();
	float RadiusSqr = 0.0f;
	for (int32 VertexIndex = 0; VertexIndex < BuildVertexData.Position.Num(); VertexIndex++)
	{
		RadiusSqr = FMath::Max(RadiusSqr, (BuildVertexData.Position[VertexIndex] - Center).SizeSquared());
	}

	MeshBounds.Origin = FVector(Center);
	MeshBounds.BoxExtent = FVector(Bounds.GetExtent());
	MeshBounds.SphereRadius = FMath::Sqrt(RadiusSqr);

	// Optimize before setting the buffer
	if (bCacheOptimize)
	{
		BuildOptimizationHelper::CacheOptimizeVertexAndIndexBuffer(BuildVertexData, OutPerSectionIndices, OutWedgeMap);
		//check(OutWedgeMap.Num() == MeshDescription->VertexInstances().Num());
	}

	RemapVerts.Empty();
}

/**
 * Utility function used inside FStaticMeshBuilder::Build() per-LOD loop to populate
 * the Sections in a FStaticMeshLODResources from PerSectionIndices, as well as
 * concatenate all section indices into CombinedIndicesOut.
 * Returned bNeeds32BitIndicesOut indicates whether max vert index is larger than max int16
 */
void BuildCombinedSectionIndices(
	const TArray<TArray<uint32>>& PerSectionIndices,
	FStaticMeshSectionArray& SectionsOut,
	TArray<uint32>& CombinedIndicesOut,
	bool& bNeeds32BitIndicesOut)
{
	bNeeds32BitIndicesOut = false;
	for (int32 SectionIndex = 0; SectionIndex < SectionsOut.Num(); SectionIndex++)
	{
		FStaticMeshSection& Section = SectionsOut[SectionIndex];
		const TArray<uint32>& SectionIndices = PerSectionIndices[SectionIndex];
		Section.FirstIndex = 0;
		Section.NumTriangles = 0;
		Section.MinVertexIndex = 0;
		Section.MaxVertexIndex = 0;

		if (SectionIndices.Num())
		{
			Section.FirstIndex = CombinedIndicesOut.Num();
			Section.NumTriangles = SectionIndices.Num() / 3;

			CombinedIndicesOut.AddUninitialized(SectionIndices.Num());
			uint32* DestPtr = &CombinedIndicesOut[Section.FirstIndex];
			uint32 const* SrcPtr = SectionIndices.GetData();

			Section.MinVertexIndex = *SrcPtr;
			Section.MaxVertexIndex = *SrcPtr;

			for (int32 Index = 0; Index < SectionIndices.Num(); Index++)
			{
				uint32 VertIndex = *SrcPtr++;

				bNeeds32BitIndicesOut |= (VertIndex > MAX_uint16);
				Section.MinVertexIndex = FMath::Min<uint32>(VertIndex, Section.MinVertexIndex);
				Section.MaxVertexIndex = FMath::Max<uint32>(VertIndex, Section.MaxVertexIndex);
				*DestPtr++ = VertIndex;
			}
		}
	}
}

} // namespace UE::Private::StaticMeshBuilder

void BuildAllBufferOptimizations(FStaticMeshLODResources& StaticMeshLOD, const FMeshBuildSettings& LODBuildSettings, TArray< uint32 >& IndexBuffer, bool bNeeds32BitIndices, const FConstMeshBuildVertexView& BuildVertices)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(BuildAllBufferOptimizations);

	if (StaticMeshLOD.AdditionalIndexBuffers == nullptr)
	{
		StaticMeshLOD.AdditionalIndexBuffers = new FAdditionalStaticMeshIndexBuffers();
	}

	const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;

	// Build the reversed index buffer.
	if (LODBuildSettings.bBuildReversedIndexBuffer)
	{
		TArray<uint32> InversedIndices;
		const int32 IndexCount = IndexBuffer.Num();
		InversedIndices.AddUninitialized(IndexCount);

		for (int32 SectionIndex = 0; SectionIndex < StaticMeshLOD.Sections.Num(); ++SectionIndex)
		{
			const FStaticMeshSection& SectionInfo = StaticMeshLOD.Sections[SectionIndex];
			const int32 SectionIndexCount = SectionInfo.NumTriangles * 3;

			for (int32 i = 0; i < SectionIndexCount; ++i)
			{
				InversedIndices[SectionInfo.FirstIndex + i] = IndexBuffer[SectionInfo.FirstIndex + SectionIndexCount - 1 - i];
			}
		}
		StaticMeshLOD.AdditionalIndexBuffers->ReversedIndexBuffer.SetIndices(InversedIndices, IndexBufferStride);
	}

	// Build the depth-only index buffer.
	TArray<uint32> DepthOnlyIndices;
	{
		BuildOptimizationHelper::BuildDepthOnlyIndexBuffer(
			DepthOnlyIndices,
			BuildVertices,
			IndexBuffer,
			StaticMeshLOD.Sections
		);

		if (DepthOnlyIndices.Num() < 50000 * 3)
		{
			BuildOptimizationThirdParty::CacheOptimizeIndexBuffer(DepthOnlyIndices);
		}

		StaticMeshLOD.DepthOnlyIndexBuffer.SetIndices(DepthOnlyIndices, IndexBufferStride);
	}

	// Build the inversed depth only index buffer.
	if (LODBuildSettings.bBuildReversedIndexBuffer)
	{
		TArray<uint32> ReversedDepthOnlyIndices;
		const int32 IndexCount = DepthOnlyIndices.Num();
		ReversedDepthOnlyIndices.AddUninitialized(IndexCount);
		for (int32 i = 0; i < IndexCount; ++i)
		{
			ReversedDepthOnlyIndices[i] = DepthOnlyIndices[IndexCount - 1 - i];
		}
		StaticMeshLOD.AdditionalIndexBuffers->ReversedDepthOnlyIndexBuffer.SetIndices(ReversedDepthOnlyIndices, IndexBufferStride);
	}

	// Build a list of wireframe edges in the static mesh.
	{
		TArray<BuildOptimizationHelper::FMeshEdge> Edges;
		TArray<uint32> WireframeIndices;

		BuildOptimizationHelper::FMeshEdgeBuilder(IndexBuffer, BuildVertices, Edges).FindEdges();
		WireframeIndices.Empty(2 * Edges.Num());
		for (int32 EdgeIndex = 0; EdgeIndex < Edges.Num(); EdgeIndex++)
		{
			BuildOptimizationHelper::FMeshEdge&	Edge = Edges[EdgeIndex];
			WireframeIndices.Add(Edge.Vertices[0]);
			WireframeIndices.Add(Edge.Vertices[1]);
		}
		StaticMeshLOD.AdditionalIndexBuffers->WireframeIndexBuffer.SetIndices(WireframeIndices, IndexBufferStride);
	}
}